1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to a data reception method and apparatus for canceling interference signals in a communication system in which interference signals exist.
2. Description of the Related Art
In the next generation communication system, extensive research is being conducted in order to provide users with high-speed services having various Qualities of Service (QoS) levels. Particularly, in the next generation communication system, active research is being carried out in order to support high-speed services in order to guarantee mobility and QoS for a Broadband Wireless Access (BWA) communication system such as a Wireless Local Area Network (WLAN) system and a Wireless Metropolitan Area Network (WMAN) system. An Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d standard based communication system and an IEEE 802.16e standard based communication system are the typical BWA communication systems.
The IEEE 802.16a/d communication system and the IEEE 802.16e communication system employ Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to support broadband transmission networks for physical channels of the WMAN system. The IEEE 802.16a/d communication system takes into account the state where a Subscriber Station (SS) is stationary, i.e. mobility of the SS is not considered at all, and a single-cell configuration. However, the IEEE 802.16e communication system takes into account mobility of the SS in the IEEE 802.16a communication system. Herein, the SS having mobility will be referred to as a “Mobile Station (MS).”
In the BWA communication system, because cells constituting the communication system share the limited resources, i.e. frequency resources, code resources, time slot resources, etc., interference may occur between the cells, especially between neighbor cells. The interference between neighbor cells is greater in a communication system using a frequency reuse factor=1. More specifically, the use of the frequency reuse factor=1 can increase efficiency of the frequency resources. In this case, however, an MS located in an interference area between neighbor cells, especially located in a cell boundary, suffers a considerable decrease in received Carrier-to-Interference and Noise Ratio (CINR) from a Base Station (BS) that manages its own cell (hereinafter referred to as a “serving BS”). In the communication system with frequency reuse factor=1, an MS located in the vicinity of the serving BS, may have no difficulty in performing communication with the serving BS due to the low interference levels. However, an MS located in the cell boundary may suffer a reduction in system performance, because it receives interferences from a BS managing neighbor cells (hereinafter referred to as a “neighbor BS”).
In order to cancel the interference between neighbor cells, during downlink communication, the MS improves a received CINR from the serving BS using an interference canceller, and, during uplink communication, the serving BS improves a received CINR from the MS using an interference canceller, thereby improving the system performance. However, in the method of improving the system performance using the interference canceller, the interference canceller cannot correctly cancel interference signals, or cannot correctly restore signals received from the serving BS, failing to meet the expected improvement of the system performance.
FIG. 1 illustrates a configuration of a general IEEE 802.16e communication system.
Referring to FIG. 1, the communication system has a multi-cell configuration, i.e. has a cell #1 110 and a cell #2 120, and includes a BS #1 111 and a BS #2 121 that manage the cell #1 110 and a cell #2 120, respectively. An MS 113 that is located in the cell #1 110, receives a communication service from the BS #1 111. It will be assumed herein that signal exchange between the BSs 111 and 121 and the MS 113 is achieved through a first channel h1 and a second channel h2 using OFDM/OFDMA.
The MS 113 is located in the boundary of the cell #1 110, and the BS #1 111 transmits data to the MS 113 located in the cell #1 110 using an A-1 frequency region 151. The BS #2 121 that manages the cell #2 120 which is a neighbor cell of the MS 113, transmits data to MSs located in the cell #2 120 through a B-1 frequency region 161 and a B-2 frequency region 163. In this case, the MS 113 located in the boundary of the cell #1 110 may receive interference due to the data transmitted by the BS #2 121 which is a neighbor BS, while receiving data from the BS #1 111 which is a serving BS through the A-1 frequency region 151.
In other words, there is an overlapping region between the A-1 frequency region 151 allocated by the BS #1 111 to the MS 113, and the B-1 frequency region 161 and the B-2 frequency region 163 allocated by the BS #2 121 to MSs located in the cell #2 120. The overlapping region becomes an interference region for the MS 113 located in the boundary of the cell #1 110. Because of the interference region, if the BS #2 121 of the cell #2 120 transmits data through the B-1 frequency region 161 and the B-2 frequency region 163 using the same time-frequency resources as those of the BS #1 111, while the MS 113 is receiving data from the BS #1 111 through the A-1 frequency region 151, then the MS 113 located in the boundary of the cell #1 110 suffers a decrease in received CINR, causing a reduction in reception performance.
In order to prevent the reduction in the CINR due to the interference from the cell #2 120, the MS 113 cancels interference using an interference canceller as described above. However, because the BS #1 111 and the BS #2 121 allocate resources independently of each other, the interference canceller may not exactly cancel interference signals, or may not accurately restore signals received from the BS #1 111, which is a serving BS, thus failing to meet the expected improvement of the system performance.
More specifically, if the BS #2 121 of the cell #2 120 transmits data through the B-1 frequency region 161 and the B-2 frequency region 163, while the MS 113 is receiving data from the BS #1 111, which is a serving BS through the A-1 frequency region 151, then the MS 113, as the data transmitted by the BS #2 121 serves as interference thereto, requires information on the overlapping region, i.e. interference region, between A-1 frequency region 151 and the B-1 frequency region 161 and B-2 frequency region 163, in order to properly cancel the interference. In addition, the MS 113 requires information on a Modulation and Coding Scheme (MCS) level of the data transmitted through the B-1 frequency region 161, and an MCS level of the data transmitted through the B-2 frequency region 163, and further requires information on the channel h2 of the cell #2 120.
In order to cancel neighbor cell interference of the cell #2 120, the MS 113 needs to estimate the channel of the cell #2 120 using MAP information of the BS #2 121 and pilots received from the BS #2 121. Accordingly, in order to cancel the neighbor cell interference, the MS 113 needs to acquire the above information, thus increasing its load and reducing the system performance. The reduction in the system performance is more considerable especially when the MS 113 has multiple neighbor cells.
FIG. 2 illustrates an operation of canceling interference signals by an MS in a general IEEE 802.16e communication system.
Referring to FIG. 2, the MS detects in step 201 an interference signal from a reception signal received via a reception antenna. Thereafter, the MS regenerates the detected interference signal in step 203, and cancels the interference signal in step 205. In step 207, the MS calculates a Log Likelihood Ratio (LLR) of the interference signal-canceled reception signal (i.e. the reception signal from which the interference signal is canceled), and delivers the LLR to a decoder.
Because such an interference cancellation scheme cancels an interference signal from a reception signal and then calculates an LLR of the interference signal-canceled reception signal, this scheme shows high performance in an area with high interference power, for example, in a low-CINR area, but shows low performance in an area with low interference power, for example, in a high-CINR area.
In addition, the key issue in communication is to transmit data through a channel efficiently and reliably. As the next generation multimedia communication system, which is now under active research, requires a high-speed communication system that can process and transmit various information such as image, radio data and the like, beyond the early voice-oriented service, it is necessary to increase the system efficiency by employing a channel coding scheme suitable to the system.
In the communication system, the wireless channel environment, unlike the wired channel environment, suffers from inevitable errors due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, interference, fading, and the like, thereby causing information loss. The information loss causes considerable distortion of the actual transmission signals, reducing the entire performance of the communication system. Generally, in order to reduce the information loss, various error control techniques are used according to channel characteristic to increase the system reliability. One of the error control techniques uses error correction codes.
In order to prevent unstable communication due to the fading, a diversity technique is used, and the diversity technique is roughly classified into a time diversity technique, a frequency diversity technique, and an antenna diversity technique, i.e. spatial diversity technique.
The antenna diversity technique, a diversity technique using multiple antennas, is classified into a reception antenna diversity technique using multiple reception antennas, a transmission antenna diversity technique using multiple transmission antennas, and a Multiple Input Multiple Output (MIMO) technique using multiple reception antennas and multiple transmission antennas.
In the MIMO-based communication system, the data to be transmitted via each of the transmission antennas is determined by Space-Time Coding (STC), and each of the reception antennas receives the signal transmitted from its associated transmission antenna and performs STC decoding on the received signal. The STC coding is implemented with a space-time transmission diversity technique for encoding the same data in different formats to transmit the data via different transmission antennas, or a spatial multiplexing technique for transmitting different data via different transmission antennas.
Generally, in the spatial multiplexing technique, an STC-coded signal is decoded in a receiver using a joint or separate detection scheme. The joint detection scheme should take into account not only the signal transmitted from one transmission antenna, but also the signals transmitted from the other transmission antennas, which serve as interference signals. Because of such characteristics, a maximum likelihood decoding technique is known as an optional decoding algorithm for using the spatial multiplexing MIMO-based communication system. The use of the maximum likelihood decoding technique can obtain a number of equal diversity orders equal to the number of reception antennas, regardless of the number of transmission antennas. Therefore, the maximum likelihood decoding technique, compared with the other decoding techniques, shows high performance in terms of a Signal-to-Noise Ratio (SNR), and its SNR gain increases with the number of transmission antennas. However, as the number of transmission antennas increases, the maximum likelihood decoding technique exponentially increases in complexity of the communication system.